Asuang: Shapechanging Horrors is a good sourcebook if you want to introduce a change-of-pace set of monsters into your campaign, especially if your players are intimately familiar with the Monster Manual and the 'acceptable variants'.
The asuang are akin to vampires (viscera-sucking rather than blood-sucking), werewolves (monsters by night, human-looking by day), and doppelgangers (not human, no matter what you might think), but have a mythology and logic and flavor of their own that can be quite dangerous if players assume they're dealing with the more common monsters.
I think that the statement early on really brings out the strength of these creatures:
Rather, people fear the asuangs because when the sun rises, an asuang can wear the face of a perfectly normal person: a merchant, priest, friend or neighbor... When a loved one is revealed as an asuang, it means that at one point, he or she was consumed painfully and replaced by the monster.
Of course, even that rule isn't closely followed by all of them. And that variety (within certain thematic similarities) makes them truly horrible foes.
Each monster has the following informational elements, in addition to the various monster stat blocks for a given asuang and its variants:
- monster name, general description, and behavior
- monster tactics -- how they monster fights when acting alone, as a mob, or as part of a mixed group
- monster mythos -- what lorekeepers believe about the creature, its beliefs, its origins, its motivations and its occasional variants
Some of my favorite bits in the book:
- the Balbal -- a version of the Mananaggal (an asuang that, at night, separates from her lower half and sprouts bat wing before going off to hunt viscera from humans) that has lost its lower half and has gone even more feral and vicious;
- the Tianak -- the undead creature created from infants and the unborn; a terrible creature to inflict on the more noble members of the party because of its tendency to appear as a child in need as a trap;
- the Tiktik -- the evil elemental demon spirit that the pre-Mananaggals must join with in a ritual to become fully asuang.
I heartily recommend picking up this sourcebook to spice up the encounters of a complacent, know-it-all adventuring party.